1. Field of the Invention
The present invention relates to audio level control for compressed data.
2. Description of the Related Art
Digital television, such as that provided by DIRECTV®, the assignee of the present invention, is typically transmitted as a digital data stream encoded using the MPEG (Motion Pictures Experts Group) standard promulgated by the ISO (International Standards Organization). MPEG provides an efficient way to represent video and audio in the form of a compressed bit stream.
The MPEG-1 standard is described in a document entitled “Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 MBit/s,” ISO/IEC 11172 (1993), which is incorporated by reference herein. The MPEG-2 standard is described in a document entitled “Generic Coding of Moving Pictures and Associated Audio Information,” ISO/IEC 13818 (1998), which is incorporated by reference herein.
Even though a satellite broadcaster, DIRECTV® provides its subscribers with local programming, i.e., local television channels, which requires that each of the television channels within a city be encoded into MPEG and statistically-multiplexed at a collection facility, before being transported via common carrier to a broadcast center for uplinking to satellites operated by DIRECTV®. Agreements can be made with other satellite broadcasters and cable operators to share these collection facilities, in order to reduce costs.
In addition, program providers, such as Disney®, Viacom®, HBO®, Showtime®, Starz®, ESPN®, etc., often provide DIRECTV® with a pre-encoded and statistically-multiplexed MPEG data stream. These program providers may ask that the MPEG data stream be passed directly through to DIRECTV® subscribers without decoding and re-encoding.
However, problems can arise in using these different MPEG data streams, due to the fact that the various satellite broadcasters, cable operators and program providers may use different standards that result in different audio levels. For example, DIRECTV® follows the SMPTE (Society of Motion Picture and Television Engineers) recommendation that a 0 dB reference level is at −20 dB from digital full scale, while other satellite broadcasters, cable operators or program providers may operate with a 0 dB reference level that is at −17 dB from digital full scale.
If these different MPEG data streams use one or more different standards, then the broadcast channels resulting therefrom will appear to be either too loud or too soft, as compared to other channels. Thus, there is a need to change the audio levels of an MPEG audio data stream.
There are additional applications where there is need for the ability to change the audio levels of an MPEG data stream. For example, television production generally runs with a wide dynamic range, providing the ability for the creative programmer to “turn up” the audio during a climax. Also, classical music often runs with a wide dynamic range.
On the other hand, most popular music has its dynamic range severely limited. This limiting of dynamic range is done for many reasons:
1) The artist desires the music to be played loudly.
2) Radio stations often believe that having silence is akin to being off the air.
3) In high-noise listening environment, such as an automobile, stadium or other public venue, it is necessary to have a narrow dynamic range to be heard over the noise.
4) The recording technology imparts a high noise level, e.g. cassette tapes, and a limited dynamic range masks the noise.
5) The playback technology has a limited dynamic range, e.g. battery-operated personal listening devices.
With regard to personal MPEG players, the 0 dB reference level for many of these devices is at −10 dB digital full scale. Consequently, if an MPEG audio data stream uses a 0 dB reference level at −20 dB digital full scale, then the volume control of the device would have to be turned up by 10 dB to compensate. However, there is limited gain range in many of these devices, since they do not support wide dynamic range audio. A better solution, then, is to change the audio levels of the MPEG audio data stream.
In the prior art, a method of altering the audio levels would comprise (1) decode (decompress) the MPEG audio data stream, (2) adjust the gain, and (3) encode (recompress) the MPEG audio data stream. This method is advantageous because commercially-available encoders and decoders may be purchased at a relatively low price. However, this method has many drawbacks, including the injection of a considerable time delay, at least 48 milliseconds (ms), as well as an increase in noise and distortion caused by yet another re-quantization of the audio.
Consequently, there is need for the ability to change audio levels of MPEG audio data streams without decompressing the audio data within the MPEG audio data streams, altering the gain levels of the audio data, and then re-compressing the audio data within the MPEG audio data streams.